The present disclosure relates to a solid-state image sensor and a signal processing method of the same, specifically a solid-state image sensor having a structure which enables a wide optical dynamic range and which is capable of reducing artifacts caused by color mixture, and a signal processing method of the same.
In typical solid-state image sensors, light from an object that is incident on a plurality of pixels arranged in a matrix is converted into signal charge to be accumulated in a photodiode included in each pixel. Once the signal charge spills over the photodiode, signals output according to the signal charge become constant. Thus, once the amount of light coming into a photodiode exceeds the saturation level of the photodiode, it is impossible to resolve the brightness of the optical wavelength band of the incoming light. Pixel size is particularly reduced in recent solid-state image sensors to reduce chip size and increase the number of pixels, and the size of a photodiode is thus reduced, too. This leads to a reduction in an optical dynamic range.
To increase the dynamic range, the following technique has been used.
FIG. 6 illustrates a plan view of an example structure of a conventional solid-state image sensor.
According to the conventional solid-state image sensor of FIG. 6, two types of pixels having different sensitivities, for example, a high sensitivity pixel 101 and a low sensitivity pixel 102, are alternately arranged in a row direction. Signal charge generated by photoelectric conversion in the two types of pixels is transferred to a vertical transfer register 103 and then a horizontal transfer register 104. The signal charge received in the horizontal transfer register 104 is further transferred from an upper horizontal transfer electrode 105 and a transfer section including an overflow drain (OFD) for charge clip, to a charge detection section 106 (an output gate electrode HOG, a floating diffusion FD, a reset gate RG, and a reset drain RD) and an amplifier section 107 to be converted into signals as voltage and output as an image signal S1 (see, for example, Japanese Laid-Open Patent Application Publication No. 11-234575).
FIG. 7 is a flowchart of signal processing of the conventional solid-state image sensor.
In the conventional solid-state image sensor of FIG. 6, a decision circuit (not shown) determines at S201 whether or not a high-sensitivity-pixel output signal T1 (a signal output from the high sensitivity pixel 101) is saturated, as illustrated in FIG. 7. Specifically, the decision circuit receives the high-sensitivity-pixel output signal T1 and determines whether or not the signal T1 is smaller than a saturated-high-sensitivity-pixel output signal T0 (a signal output from the saturated high sensitivity pixel 101). If the signal T1 is greater than the signal T0 (“No” at S201), in other words, if the signal T1 is saturated, the processing goes to S202. At S202, an adder circuit (not shown) adds together a low-sensitivity-pixel output signal T2 (a signal output from the low sensitivity pixel 102) and a threshold value that is smaller than the signal T1 to generate and output a pixel signal T3. On the other hand, if the signal T1 is smaller than the signal T0 (“Yes” at S201), in other words, if the signal T1 is not saturated, the processing goes to S203. At S203, an adder circuit (not shown) adds the signal T1 and the signal T2 together to generate and output a pixel signal T4.
As explained in the above, the signal processing method of the conventional solid-state image sensor can achieve a wide dynamic range even in the case where the signal charge of the high sensitivity pixel 101 is saturated.
The common techniques for forming the two types of pixels having different sensitivities in the conventional solid-state image sensor include, for example, making pixels arranged next to each other have photodiodes different in size, and making shielding films formed above photodiodes have different aperture sizes.